1. Field of the Invention
The present invention relates to a magnetic disk apparatus provided with a rotating actuator and particularly to a magnetic disk apparatus provided with a rotating actuator in which the influence of thermal expansion is reduced so that high-density recording may be performed.
2. Description of the Related Art
As shown in FIG. 1, generally, a magnetic disk apparatus is provided with a magnetic disk 5 for recording and reproducing data, a magnetic head 4 provided at an end of a carriage 3 pivoted on a pivot bearing 2, and a voice coil motor (VCM) having a magnet 6 called actuator for controlling Lorentz force by use of a current flowing in a coil 7 to thereby rotate the carriage 3. The magnet 6 rotates the carriage 3 so that data recording/reproducing may be performed in the condition that the magnetic head 4 is positioned above an optional track on the magnetic disk 5.
The controllability of the actuator is affected by a resonance frequency inherent in the structure of the actuator. It is known that the controllability is enhanced more greatly as lowest resonance frequency (main resonance frequency) in a pivotal moving direction which is a controlling direction becomes higher.
The structure of the actuator is as shown in FIGS. 2A and 2B. The actuator is constituted of: a columnar shaft 23 serving as a center of rotation; a cylindrical sleeve 21; a pair of bearing units 22 having outer races 222 fitted to upper and lower positions of the sleeve 21, and inner races 221 fitted to the outer side of the shaft 23 through bearing balls 223; and a carriage 3 fitted to the sleeve 21 and having one end connected to the magnetic head, and the other end connected to a coil.
As shown in FIG. 2B which is an enlarged view of a portion A of FIG. 2A, each of the bearing units 22 has bearing balls 223 put in both recesses provided in the inside of the bearing outer and inner races 222 and 221. Each of the bearing units 22 is attached in the condition that the bearing outer race 222 is preloaded with a pressure P acting upward in FIG. 2B.
Because the stiffness of the bearing unit 22 becomes lowest in a pivotal moving direction of the actuator configured as described above, the characteristic frequency of a spring-mass system having the stiffness of the bearing unit 22 as a spring and mass of a movable portion of the actuator as mass serves as a main resonance frequency. This can be expressed as a model shown in FIG. 3.
As shown in FIG. 3, when the stiffness of the shaft 23 connecting a spring and a stationary portion to each other is sufficiently high, the characteristic frequency f is given by the expression:
xe2x80x83f=1/2/xcfx80{square root over ((k/m))}
in which k is a spring constant of the spring, and m is a mass of the movable portion of the actuator. Accordingly, it is understood that the controllability of the actuator can be enhanced by reduction of the mass m, that is, by reduction of the mass of the movable portion of the actuator. On the other hand, when the stiffness of the shaft 23 is low, the stiffness of the shaft 23 has influence on the main resonance frequency.
Generally, parts (races, rollers, etc.) of the bearing unit 22 used for the magnetic disk apparatus are in most cases made of stainless steel from the point of view of stiffness and corrosion resistance. Heretofore, the same material as that of the bearing unit 22 is applied to the sleeve 21 and the shaft 23 in order to avoid the change of the pre-load due to the change of the temperature of the pivot bearing 2 in accordance with the difference in thermal expansion coefficient between the sleeve 21 and the bearing unit 22 and between the sleeve 21 and the shaft 23.
The structure of the actuator has characteristic as follows. When the pre-load decreases, the stiffness of the bearing unit 22 (that is, the pressing force of the bearing balls 223 against the inner and outer races) becomes so low that the main resonance frequency is reduced. When the pre-load contrariwise increases, the life of the bearing unit 22 is shortened (because the pressing force of the bearing balls 223 against the inner and outer races becomes high). Hence, it is, known that the performance of the actuator is lowered by the change of the pre-load applied to the bearing unit 22. Accordingly, a material lighter in specific gravity than stainless steel was unable to be used as the material of the sleeve 21 which was a part of the movable portion of the actuator.
JP-A-8-93758 is a literature concerning the rotating actuator and has described a technique of positively setting the linear expansion coefficient of the sleeve to be larger than the linear expansion coefficient of the shaft to thereby increase the pre-load applied to the bearing unit at the rise of the temperature. In the technique, however, there was no consideration of reduction of the main resonance frequency and shortening of the life of the bearing unit though there was an effect of cancelling reduction of rotation torque due to the viscosity change of grease in the bearing unit at the temperature change.
As described above, in the rotating actuator, when the sleeve of the pivot bearing was made of a material different in kind from that of the shaft, the change of the pre-load applied to the bearing unit at the temperature change was caused by the difference between linear expansion coefficients due to the difference in kind between the materials. Hence, there was a disadvantage in that reduction of controllability was caused by reduction of stiffness when, for example, the pre-load decreased and in that the life of the bearing unit was shortened when, for example, the pre-load increased. Hence, a material lower in specific gravity than stainless steel was unable to be used as the material of the sleeve in the rotating actuator. Hence, there was also a disadvantage in that it was difficult to lighten the weight of the rotating actuator as a whole to enhance controllability.
An object of the present invention is to eliminate the disadvantages from the aforementioned technique, that is, to provide a high-accuracy and large-capacity magnetic disk apparatus in which the weight of a pivotal moving portion of an actuator is reduced to enhance the head-positioning controllability of the actuator.
In order to achieve the above object, according to a first aspect of the present invention, there is provided with a magnetic disk apparatus comprising a magnetic disk, a magnetic head for writing data into and reading data from the magnetic disk, and a carriage pivoted on a pivot bearing while holding the magnetic head at its one end, the pivot bearing including a cylindrical sleeve serving as a fixed axle, bearing units each having an outer race fitted to the inside of the sleeve, bearing balls, and an inner race, and a shaft having the outside fitted to the inner races of the bearing units, wherein: a pre-load in an axial direction of the shaft is applied between the inner and outer races in each of the bearing units; and the specific gravity of the sleeve is set to be lower than that of the shaft.
According to a second aspect of the present invention, in the aforementioned magnetic disk apparatus, a pre-load in an axial direction of the shaft is applied between the inner and outer races in each of the bearing units; and the shaft and the sleeve are configured so that the quantity of the change of the pre-load due to the difference in thermal expansion between the shaft and the sleeve in the axial direction of the shaft cancels the quantity of the change of the pre-load due to the difference in thermal expansion between the shaft and sleeve in a radial direction of the shaft.
According to a third aspect of the present invention, in the aforementioned magnetic disk apparatus, the Young""s modulus of the shaft of the pivot bearing is set to be larger than that of the sleeve. Further, according to a fourth aspect of the present invention, the shaft of the pivot bearing is made of stainless steel whereas the sleeve is made of an aluminum type material.